Hydrophilic porous carbon electrode and manufacturing method of same

ABSTRACT

A hydrophilic porous carbon electrode which has excellent hydrophilicity, which has high reaction activity when used for a battery, and with which excellent battery characteristics is able to be obtained is provided. A hydrophilic porous carbon electrode is a sheet-form hydrophilic porous carbon electrode in which a carbon fiber is bonded using a resin carbide and has a contact angles θA of water on both surfaces in a thickness direction being 0 to 15° and a contact angle θB of water in a middle portion in the thickness direction being 0 to 15°. The hydrophilic porous carbon electrode is obtained by forming the carbon fiber and a binder fiber into a sheet, impregnating the sheet into a thermosetting resin, subjecting it to heat press processing, and then subjecting it to carbonization at 400 to 3000° C. in an inert atmosphere. The hydrophilic porous carbon electrode is transported and is subjected to a heat treatment while an oxidizing gas flows at 400 to 800° C. in a direction perpendicular to a direction in which the hydrophilic porous carbon electrode is transported to be subjected to hydrophilization.

The present invention relates to a hydrophilic porous carbon electrodeand a manufacturing method of the same.

This application is a continuation application of InternationalApplication No. PCT/JP2019/005027, filed on Feb. 13, 2019, which claimsthe benefit of priority of the prior Japanese Patent Application No.2018-025086, filed Feb. 15, 2018 and Japanese Patent Application No.2018-025411, filed Feb. 15, 2018, the content of which is incorporatedherein by reference.

BACKGROUND ART Technical Field

For electrodes of batteries such as lithium ion batteries, fuel cells,and redox flow batteries, porous base materials in which carbon fibershaving a short fiber length are dispersed and bonded with resin carbidesare widely used. Among these batteries, in redox flow batteries in whichsizes thereof may easily become larger and which have excellentdurability, the porous base materials are used at they are as porouscarbon electrodes.

Redox flow batteries usually include external tanks configured to storeelectrolytic solutions and electrolytic cells and electrochemical energyconversion, that is, charging/discharging is performed using electrodesincorporated in the electrolytic cells while electrolytic solutionscontaining active materials are transferred from the external tanks tothe electrolytic cells through pumps. In order to obtain excellentbattery characteristics in redox flow batteries, it is important thatporous carbon electrodes provided in the electrolytic cells haveexcellent hydrophilicity and high reaction activity.

As a porous carbon electrode having enhanced hydrophilicity, PatentDocument 1 describes a porous carbon electrode in which carbon fineparticles having conductivity are adhered to a carbon fiber using an ionconductive binder. Patent Document 2 describes a porous carbon electrodeusing a carbon fiber whose surface is treated using an alkalinedegreasing liquid. Patent Document 3 describes a porous carbon electrodeusing a carbon fiber to which a metal oxide is attached.

CITATION LIST Patent Literature

[Patent Document 1]

Japanese Unexamined Patent Application, First Publication No. 2017-27920

[Patent Document 2]

Japanese Unexamined Patent Application, First Publication No.2007-186823

[Patent Document 3]

Published Japanese Translation No. 2005-507980 of the PCT InternationalPublication

SUMMARY OF INVENTION Technical Problem

However, in redox flow batteries using conventional porous carbonelectrodes as in Patent Documents 1 to 3, the battery characteristicsare still insufficient and there is a demand for higher performanceporous carbon electrodes.

An object of the present invention is to provide a hydrophilic porouscarbon electrode which has excellent hydrophilicity, high reactionactivity when used for a battery, and excellent battery characteristicsand a manufacturing method of the same.

Solution to Problem

The present invention has the following constitution.

[1] A hydrophilic porous carbon electrode which is a sheet-like porouscarbon electrode in which carbon fibers are bonded using a resin carbideand has contact angles θ_(A) of water with respect to both surfaces ofthe hydrophilic porous carbon electrode in a thickness direction of 0 to15° and a contact angle θ_(B) of water in a middle portion of thehydrophilic porous carbon electrode in the thickness direction of 0 to15°.[2] In the hydrophilic porous carbon electrode according to [1], thecontact angles θ_(A) of water on both surfaces of the hydrophilic porouscarbon electrode in the thickness direction may be 0 to 10° and thecontact angle θ_(B) of water in the middle portion of the hydrophilicporous carbon electrode in the thickness direction may be 0 to 15°.[3] In the hydrophilic porous carbon electrode according to [1] or [2],differences between the contact angles θ_(A) of water on both surfacesof the hydrophilic porous carbon electrode in the thickness directionand the contact angle θ_(B) of water in the middle portion of thehydrophilic porous carbon electrode in the thickness direction may be 0to 10°.[4] In the hydrophilic porous carbon electrode according to any one of[1] to [3], the basis weight is 30 to 300 g/m² and the thickness is 0.10to 0.80 mm.[5] In the hydrophilic porous carbon electrode according to [4], thespecific surface area is 1.0 to 1000 m²/g.[6] In the hydrophilic porous carbon electrode according to [5],non-through holes with a size of an opening portion of 0.1 to 5.0 μm anda depth of 0.01 to 1.0 μm are formed in surfaces of the carbon fibers.[7] In the hydrophilic porous carbon electrode according to [5] or [6],Ag is present on a surface of the porous carbon electrode and apenetration resistance is 3.0 to 6.0 mΩ·cm².[8] In the hydrophilic porous carbon electrode according to [5], eitheror both of a Ti oxide and a Sn oxide are present on the surface of theporous carbon electrode.[9] A manufacturing method of the hydrophilic porous carbon electrodeaccording to any one of [1] to [8] includes: the following Steps (1) to(5): Step (1): a step of forming carbon fibers and a binder fiber into asheet to obtain a carbon fiber sheet; Step (2): a step of impregnatingthe carbon fiber sheet with a thermosetting resin to obtain aresin-impregnated carbon fiber sheet; Step (3): a step of subjecting theresin-impregnated carbon fiber sheet to heat press processing to obtaina resin-cured carbon fiber sheet; Step (4): a step of subjecting theresin-cured carbon fiber sheet to carbonization at 400 to 3000° C. in aninert atmosphere to obtain a porous carbon electrode; and Step (5): astep of carrying out a hydrophilization treatment by transporting theporous carbon electrode and subjecting the porous carbon electrode to aheat treatment while causing an oxidizing gas at 400 to 800° C. to flowin a direction perpendicular to a direction in which the porous carbonelectrode is transported.[10] In the manufacturing method of the hydrophilic porous carbonelectrode according to [9], the porous carbon electrode is subjected toa heat treatment for 0.5 to 60 minutes using the oxidizing gas.[11] In the manufacturing method of the hydrophilic porous carbonelectrode according to [9] or [10], in Step (5), the porous carbonelectrode is further immersed in a nitric acid solution, subjected to anelectrolytic treatment, washed with water, and dried.[12] In the manufacturing method of the hydrophilic porous carbonelectrode according to any one of [9] to [11], in Step (5), bothsurfaces of the porous carbon electrode are further subjected to anatmospheric pressure plasma treatment.[13] In the manufacturing method of the hydrophilic porous carbonelectrode according to any one of [9] to [12], in Step (4), the porouscarbon electrode is further impregnated with a dispersion liquid of ametal oxide to support the metal oxide.

Another aspect of the present invention has the following constitution.

[A1] A hydrophilic porous carbon electrode is a sheet-like porous carbonelectrode in which carbon fibers are bonded using a resin carbide andhas contact angles of water on both surfaces of the hydrophilic porouscarbon electrode in a thickness direction being 0 to 60° and a contactangle of water in a middle portion of the hydrophilic porous carbonelectrode in the thickness direction being 0 to 60°.[A2] In the hydrophilic porous carbon electrode according to [A1], thecontact angles of water on both surfaces of the hydrophilic porouscarbon electrode in the thickness direction are 0 to 40° and the contactangle of water in the middle portion of the hydrophilic porous carbonelectrode in the thickness direction is 0 to 60°.[A3] In the hydrophilic porous carbon electrode according to [A1] or[A2], differences between the contact angles of water on both surfacesof the hydrophilic porous carbon electrode in the thickness directionand the contact angle of water in the middle portion of the hydrophilicporous carbon electrode in the thickness direction are 0 to 20°.[A4] In the hydrophilic porous carbon electrode according to any one of[A1] to [A3], the basis weight is 50 to 200 g/m² and the thickness is0.150 to 0.600 mm.[A5] A manufacturing method of a hydrophilic porous carbon electrode isa manufacturing method of the hydrophilic porous carbon electrodeaccording to any one of [A1] to [A4] and has the following Steps (A1) to(A5): Step (A1): a step of forming carbon fibers and a binder fiber intoa sheet to obtain a carbon fiber sheet; Step (A2): a step ofimpregnating the carbon fiber sheet with a thermosetting resin to obtaina resin-impregnated carbon fiber sheet; Step (A3): a step of subjectingthe resin-impregnated carbon fiber sheet to heat press processing toobtain a resin-cured carbon fiber sheet; Step (A4): a step of subjectingthe resin-cured carbon fiber sheet to carbonization at 400 to 3000° C.in an inert atmosphere to obtain a hydrophilic porous carbon electrode;and Step (A5): a step of subjecting the porous carbon electrode to ahydrophilization treatment to obtain a hydrophilic porous carbonelectrode.[A6] In the manufacturing method of the hydrophilic porous carbonelectrode according to [A5], as the hydrophilization treatment, theporous carbon electrode is transported, caused to pass in a furnace at400 to 800° C. in an air atmosphere, and subjected to a heat treatmentfor 0.5 to 60 minutes while causing a gas to flow in a directionperpendicular to a direction in which the porous carbon electrode istransported in the furnace.[A7] In the manufacturing method of the hydrophilic porous carbonelectrode according to [A5] or [A6], as the hydrophilization treatment,the porous carbon electrode is immersed in a nitric acid solution,subjected to an electrolytic treatment, washed with water, and dried.[A8] In the manufacturing method of the hydrophilic porous carbonelectrode according to [A6] or [A7], as the hydrophilization treatment,either or both of the heat treatment and the electrolytic treatment andan atmospheric pressure plasma treatment performed on both surfaces ofthe hydrophilic porous carbon electrode are performed.

Still another aspect of the present invention has the followingconstitution.

[B1] A porous carbon electrode which is a sheet-like porous carbonelectrode in which carbon fibers are bonded using a resin carbide andhas a thickness of 0.1 to 0.8 mm, a basis weight of 30 to 300 g/m², anda specific surface area of 1.0 to 1000 m²/g.

[B2] In the porous carbon electrode according to [B1], non-through holeswith a size of an opening portion of 0.1 to 5.0 μm and a depth of 0.01to 1.0 μm are formed in the carbon fibers.

[B3] In the porous carbon electrode according to [B1] or [B2], Ag ispresent on a surface of the porous carbon electrode and a penetrationresistance is 3.0 to 6.0 mΩ·cm².

[B4] In the porous carbon electrode according to [B1], either or both ofa Ti oxide and a Sn oxide are present on the surface of the porouscarbon electrode.

[B5] In the porous carbon electrode according to [B3] or [B4], contactangles of water on both surfaces of the porous carbon electrode in athickness direction and a contact angle of water in a middle portion ofthe porous carbon electrode in the thickness direction are all 0 to 60°.[B6] A manufacturing method of a porous carbon electrode which is amanufacturing method of the porous carbon electrode according to any oneof [B1] to [B5] and has the following Steps (B1) to (B6):

Step (B1): a step of forming carbon fibers and a binder fiber into asheet to obtain a carbon fiber sheet; Step (B2): a step of impregnatingthe carbon fiber sheet in a thermosetting resin to obtain aresin-impregnated carbon fiber sheet; Step (B3): a step of subjectingthe resin-impregnated carbon fiber sheet to heat press processing toobtain a resin-cured carbon fiber sheet; Step (B4): a step of subjectingthe resin-cured carbon fiber sheet to carbonization at 400 to 3000° C.in an inert atmosphere to obtain a hydrophilic porous carbon electrode;

Step (B5): a step of impregnating the porous carbon sheet in adispersion liquid of a metal oxide to obtain a metal oxide-supportingporous carbon sheet; and

Step (B6): a step of transporting the metal oxide-supporting porouscarbon sheet to pass in a furnace at 400 to 800° C. in an air atmosphereand subjecting the metal oxide-supporting porous carbon sheet to a heattreatment for 0.5 to 60 minutes while causing a gas to flow in adirection perpendicular to a direction in which the metaloxide-supporting porous carbon sheet is transported in the furnace toobtain a porous carbon electrode.

Advantageous Effects of Invention

A hydrophilic porous carbon electrode of the present invention hasexcellent hydrophilicity, high reaction activity when used for abattery, and excellent battery characteristics.

According to a manufacturing method of a hydrophilic porous carbonelectrode of the present invention, it is possible to manufacture ahydrophilic porous carbon electrode which has excellent hydrophilicity,high reaction activity when used for a battery, and excellent batterycharacteristics.

DESCRIPTION OF EMBODIMENTS

A hydrophilic porous carbon electrode (hereinafter referred to as a“carbon electrode”) of the present invention is a sheet-like porouscarbon electrode in which carbon fibers are bonded using a resincarbide. In a carbon electrode, a plurality of carbon fibers are bondedusing a resin carbide in a state in which the carbon fibers aredispersed in a sheet so that fiber directions thereof are randomlyoriented. That is to say, the carbon electrode of the present inventionis a carbon electrode formed of carbon paper.

The carbon electrode of the present invention can be suitably used for aredox flow battery.

Examples of the carbon fibers include polyacrylonitrile (PAN)-basedcarbon fibers, pitch-based carbon fibers, and rayon-based carbon fibers.Among these, PAN-based carbon fibers are preferable. As the carbonfibers, one kind may be used independently or a combination of two ormore kinds may be used.

The average fiber length of the carbon fibers is preferably 2 to 30 mm,and more preferably 2 to 12 mm. That is to say, it is desirable that thecarbon fibers be short carbon fibers. If the average fiber length of thecarbon fibers is equal to or larger than the lower limit value of theabove ranges, it is possible to easily obtain sufficient strength. Ifthe average fiber length of the carbon fibers is equal to or smallerthan the upper limit value of the above ranges, excellent dispersibilityof the carbon fibers is provided.

The average fiber length of the carbon fibers is obtained by observingthe carbon fibers through a microscope such as a scanning electronmicroscope at a magnification of 50 times or more, measuring fiberlengths of 50 short fibers which are randomly selected, and obtaining anaverage of the fiber lengths.

The average fiber diameter of the carbon fibers is preferably 3 to 20μm, and more preferably 3 to 9 μm. If the average fiber diameter of thecarbon fibers is equal to or larger than the lower limit value of theabove range, excellent dispersibility of the carbon fibers is provided.Thus, a carbon fiber sheet which is uniform in surface direction can beobtained. If the average fiber diameter of the carbon fibers is equal toor smaller than the upper limit value of the above range, a carbon fibersheet having high smoothness can be obtained.

The average fiber diameter of the carbon fibers is obtained by observingcarbon fiber cross sections through a microscope such as a scanningelectron microscope at a magnification of 50 times or more, measuringfiber diameters of 50 single fibers which are randomly selected, andobtaining an average of values of these fiber diameters. In the case ofa carbon fiber having a flat cross section, that is, when the crosssection has a longer axis and a shorter axis, the longer axis is thefiber diameter of the fiber.

The tensile elastic modulus of the carbon fibers is preferably 200 to600 GPa, and more preferably 200 to 450 GPa.

The tensile elastic modulus of the carbon fibers is obtained through asingle fiber tensile test. In the single fiber tensile test, one singlefiber is taken out from the carbon fibers and an elastic modulus of thesingle fiber is measured using a universal testing machine under testconditions such as a test length of 5 mm and a pulling rate of 0.5mm/min. A value obtained by selecting 50 single fibers from the samecarbon fibers, measuring elastic moduli thereof, and obtaining anaverage of values of the elastic moduli is assumed to be a tensileelastic modulus of the carbon fibers.

The tensile strength of the carbon fibers is preferably 3000 to 7000GPa, and more preferably 3500 to 6500 GPa.

The tensile strength of the carbon fibers is obtained through a singlefiber tensile test. In the single fiber tensile test, one single fiberis taken outside of the carbon fibers and the strength of the singlefiber is measured using a universal testing machine under testconditions such as a test length 5 mm and a pulling rate of 0.5 mm/min.A value obtained by selecting 50 single fibers from the same carbonfibers, measuring strengths thereof, and obtaining the average of valuesof the strengths is assumed to be a tensile strength of the carbonfibers.

The carbon fiber is obtained by impregnating a bundle of, for example,several thousand to tens of thousands of carbon fiber filaments with asizing agent and continuously or discontinuously cutting a dried andbundled carbon fiber bundle using a roving cutter, a guillotine cutter,or the like into predetermined lengths.

It is desirable that non-through holes having opening sizes of 0.1 to5.0 μm and depths of 0.01 to 1.0 μm be formed in a surface of the carbonfiber. Thus, since the specific surface area of the carbon electrode isfurther increased, the carbon electrode has high reaction activity whenused for a battery and superior battery characteristics. Suchnon-through holes are formed in a surface of the carbon fiber, forexample, by subjecting a porous carbon electrode having Ag₂O supportedtherein to a heat treatment which will be described later.

The opening sizes of the non-through holes are 0.1 to 5.0 μm, preferably0.1 to 4.0 μm, and more preferably 0.1 to 3.0 μm. If the opening sizesare equal to or larger than the lower limit value of the above range,the specific surface area of the carbon electrode is further increasedand the reaction activity when the carbon electrode is used for abattery is improved. If the opening sizes are equal to or smaller thanthe upper limit value of the above range, a carbon electrode havingsufficient strength can be obtained.

The opening sizes of the non-through holes refer to a diameter of acircumscribed circle of the opening portion when the opening portion isviewed from in front.

The depths of the non-through holes are 0.01 to 1.0 μm, preferably 0.05to 1.0 μm, and more preferably 0.1 to 1.0 μm. If the depths of thenon-through holes are equal to or larger than the lower limit value ofthe above range, the specific surface area of the carbon electrode isfurther increased and the reaction activity when the carbon electrode isused for a battery is improved. If the depths of the non-through holesare equal to or smaller than the upper limit value of the above range, acarbon electrode having sufficient strength can be obtained.

The depths of the non-through holes refer to depths of the non-throughholes corresponding to the deepest portions thereof.

The basis weight of the carbon fiber in the carbon electrode ispreferably 10 to 140 g/m², and more preferably 25 to 100 g/m². If thebasis weight of the carbon fiber is equal to or larger than the lowerlimit value of the above range, a carbon electrode having sufficientstrength can be obtained. If the basis weight of the carbon fiber isequal to or smaller than the upper limit value of the above range, acarbon electrode in which carbon fibers are uniformly dispersed can beobtained.

The amount of the carbon fibers in the carbon electrode is preferably 40to 80% by mass, and more preferably 50 to 70% by mass with respect tothe total mass of the carbon electrode. If the amount of the carbonfibers is equal to or larger than the lower limit value of the aboverange, it is possible to easily obtain sufficient strength. If theamount of the carbon fibers is equal to or smaller than the upper limitvalue of the above range, the amount of the resin carbide is increasedrelatively. Thus, the carbon fibers are sufficiently bonded to eachother.

The resin carbide is obtained by carbonizing a resin. A resin carbideobtained by subjecting a binder resin or a binder fiber to be used whenthe carbon electrode is manufactured to a carbonization treatment iscontained in a carbon electrode as a resin carbide.

As the binder resin, a binder resin having a binding force with thecarbon fiber and subjected to carbonization may be used. In addition,examples of the binder resin include thermosetting resins such asphenolic resins and furan resins. A binder resin may be usedindependently or a combination of two or more of binder resins may beused.

Examples of the phenolic resins include resol type phenolic resinsobtained through the reaction of phenols and aldehydes in the presenceof an alkali catalyst. Resins obtained by dissolving and mixing anovolac type solid phenolic resin having heat-sealing propertiesgenerated through the reaction of phenols and aldehydes in the presenceof an acidic catalyst into a resol type fluid phenolic resin may beused. In this case, a use of a self-crosslinking type containing, forexample, hexamethylenediamine as a curing agent is preferable.

A commercially available product may be used as the phenolic resin.

Examples of the phenols include phenol, resorcin, cresol, and xylol. Asthe phenols, one kind may be used independently or a combination of twoor more kinds may be used.

Examples of the aldehydes include formalin, paraformaldehyde, andfurfural. One kind of aldehyde may be used independently or acombination of two or more kinds of aldehyde may be used.

As the phenolic resin, a water-dispersible phenolic resin or awater-soluble phenolic resin may be used.

Examples of the water-dispersible phenolic resin includewater-dispersible phenolic resins referred to as resol type phenolicresin emulsions or aqueous dispersions described in Japanese UnexaminedPatent Application, First Publication No. 2004-307815, JapaneseUnexamined Patent Application, First Publication No. 2006-56960, and thelike.

Examples of the water-soluble phenolic resin include the resol typephenolic resins having good solubility in water which are described inJapanese Unexamined Patent Application, First Publication No. 2009-84382and the like.

The amount of the resin carbide in the carbon electrode is preferably 20to 60% by mass, and more preferably 25 to 50% by mass with respect tothe total mass of the carbon electrode. If the amount of the resincarbide is equal to or larger than the lower limit value of the aboveranges, the carbon fibers are sufficiently bonded to each other. If theamount of the resin carbide is equal to or smaller than the upper limitvalue of the above ranges, the amount of the carbon fibers is increasedrelatively. Thus, it is possible to easily obtain sufficient strength.

The carbon electrode may contain components other than carbon fibers anda resin carbide as required. As the other components, carbon powder isan exemplary example. When the carbon electrode contains carbon powder,it can be expected that there will be an improvement in conductivity.

Examples of carbon powder include graphite powder, carbon black, milledfibers, carbon nanotubes, carbon nanofibers, coke, activated carbon, andamorphous carbon. One kind of carbon powder may be used independently ora combination of two or more kinds of carbon powder may be used.

The graphite powder has a highly crystalline graphite structure and anaverage particle size of primary particles thereof is generally severalμm to several hundreds μm.

Examples of the graphite powder include pyrolytic graphite, spheroidalgraphite, scaly graphite, lump graphite, earthy graphite, artificialgraphite, and expanded graphite and pyrolytic graphite, spheroidalgraphite, or scaly graphite is preferable in view of exhibitingconductivity.

Carbon black is generally present as a structured material (anagglomerate) in which primary particles having an average particle sizeof several tens of μm are fused to each other to form structures and thestructures are joined through van der Waals forces. Carbon black has asignificantly larger number of particles per unit mass than that ofgraphite powder and agglomerates therein are connected in athree-dimensional network to form macroscopic conductive paths at acertain critical concentration or higher.

Examples of carbon black include acetylene black, Ketjen black, furnaceblack, channel black, lamp black, and thermal black.

The milled fiber may be crushed virgin carbon fibers or may be a crushedproduct of a recycled product such as a carbon fiber reinforcedthermosetting resin molded product, a carbon fiber reinforcedthermoplastic resin molded product, and a prepreg. The carbon fiber as araw material of the milled fiber may be PAN-based carbon fibers,pitch-based carbon fibers, or rayon-based carbon fibers.

When the carbon electrode contains carbon powder, the amount of thecarbon powder in the carbon electrode is preferably 1 to 40% by mass,and more preferably 5 to 30% by mass with respect to the total mass ofthe carbon electrode. If the amount of carbon powder is equal to orlarger than the lower limit value of the above range, the conductivityis improved, the specific surface area of the carbon electrode isincreased due to surface irregularities derived from the carbon powder,and the reaction activity is improved. If the amount of carbon powder isequal to or smaller than the upper limit value of the above range, it isdifficult to close a path along which the electrolytic solutiondiffuses.

The carbon electrode of the present invention has contact angles θ_(A)of water on both surfaces in a thickness direction of 0 to 15° and acontact angle θ_(B) of water in a middle portion in the thicknessdirection of 0 to 15°. When all of the contact angles θ_(A) and thecontact angle θ_(B) of the carbon electrode are 0 to 15°, the reactionactivity when the carbon electrode is used for a battery is increasedand excellent battery characteristics can be obtained.

The middle portion of the carbon electrode refers to a portion between40 and 60% of the total thickness of the carbon electrode from a surfaceof the carbon electrode in the thickness direction of the carbonelectrode.

The contact angles θ_(A) of the carbon electrode are 0 to 15°,preferably 0 to 13°, more preferably 0 to 10°, and still more preferably0 to 5°. If the contact angles θ_(A) are equal to or larger than theseupper limit values, excellent hydrophilicity is provided, high reactionactivity of the electrode is provided, and excellent batterycharacteristics can be obtained. The contact angles θ_(A) of bothsurfaces of the carbon electrode may be the same or different.

The contact angles θ_(A) of water in the carbon electrode are measuredusing a commercially available contact angle measuring device.

The contact angle θ_(B) of the carbon electrode is 0 to 15°, preferably0 to 13°, more preferably 0 to 10°, and still more preferably 0 to 5°.If the contact angle θ_(B) is equal to or smaller than the upper limitvalue, the electrolytic solution easily enters into the inside of theelectrode, the reaction activity is increased also inside the electrode,and excellent battery characteristics can be obtained.

The contact angle θ_(B) of water in the carbon electrode is measuredusing a commercially available contact angle measuring device.

In the present invention, it is desirable that the contact angles θ_(A)be 0 to 10° and the contact angle θ_(B) be 0 to 15°. In this case, thereaction activity of the electrode is further increased and superiorbattery characteristics can be obtained.

Differences between the contact angles θ_(A) and the contact angle θ_(B)of the carbon electrode are preferably 0 to 10°, more preferably 0 to5°, and still more preferably 0 to 2°. If the difference between thecontact angles θ_(A) and the contact angle θ_(B) is equal to or smallerthan the upper limit value, the reaction activity is increased over theentire electrode in the thickness direction and more excellent batterycharacteristics can be obtained.

In the present invention, it is particularly desirable that differencesbetween both of the contact angles θ_(A) of both surfaces of the carbonelectrode in the thickness direction and the contact angle θ_(B) bewithin the above range.

It is desirable that the carbon electrode of the present invention havea thickness of 0.10 to 0.80 mm and a basis weight of 30 to 300 g/m² andit is more desirable that a specific surface area be 1.0 to 1000 m²/g.When the thickness, the basis weight, and the specific surface area ofthe carbon electrode are controlled such that they are within the aboveranges, the reaction activity when the carbon electrode is used for abattery is further increased and more excellent battery characteristicscan be obtained.

The specific surface area of the carbon electrode is preferably 1.0 to1000 m²/g, more preferably 1.0 to 600 m²/g, still more preferably 1.0 to400 m²/g, particularly preferably 1.0 to 200 m²/g, and most preferably1.0 to 100 m²/g. If the specific surface area is equal to or larger thanthe lower limit value of the above range, the reaction activity of theelectrode is improved. If the specific surface area is equal to orsmaller than the upper limit value of the above range, the electrolyticsolution is easily adopted and battery characteristics are improved.

The specific surface area of the carbon electrode is measured using amercury intrusion method.

The basis weight of the carbon electrode is preferably 30 to 300 g/m²,more preferably 40 to 280 g/m², still more preferably 50 to 200 g/m²,and particularly preferably 50 to 130 g/m². If the basis weight of thecarbon electrode is equal to or larger than the lower limit value of theabove range, high sheet strength is provided, excellent handleability isprovided, and the carbon electrode functions satisfactorily as anelectrode. If the basis weight of the carbon electrode is equal to orsmaller than the upper limit value of the above range, a uniform carbonelectrode with less unevenness of the basis weight can be obtained andhigh productivity is provided.

The thickness of the carbon electrode is preferably 0.10 to 0.80 mm,more preferably 0.10 to 0.70 mm, still more preferably 0.15 to 0.60 mm,and particularly preferably 0.19 to 0.60 mm. If the thickness of thecarbon electrode is equal to or larger than the lower limit value of theabove range, a carbon electrode having a sufficient reaction surfacearea can be obtained. If the thickness of the carbon electrode is equalto or smaller than the upper limit value of the above range, a carbonelectrode with less pressure loss when the electrolytic solution istransferred can be obtained. Furthermore, high productivity is providedbecause the carbon electrode can be rolled up in a roll shape.

In the present invention, it is desirable that Ag be present on asurface of the porous carbon electrode and a penetration resistance be3.0 to 6.0 mΩ·cm². That is to say, it is desirable that Ag be present onthe carbon fiber and the resin carbide which form the porous carbonelectrode and the penetration resistance be controlled in the aboverange. Thus, the conductivity of the carbon electrode and the batterycharacteristics when the carbon electrode is used for a battery isfurther improved.

In the porous carbon electrode, Ag may be present uniformly in thethickness direction or Ag may be unevenly distributed in a surfacelayer. For example, as described above, when a porous carbon sheet isimpregnated in a dispersion liquid having Ag₂O dispersed therein andsubjected to a heat treatment, it is possible to deposit Ag on surfacesof the carbon fiber and the resin carbide of the porous carbonelectrode.

The penetration resistance of the carbon electrode whose surface havingAg present thereon is preferably 3.0 to 6.0 mΩ·cm², more preferably 3.0to 5.8 mΩ·cm², still more preferably 3.0 to 5.6 mΩ·cm². If thepenetration resistance is equal to or larger than the lower limit valueof the above range, a carbon electrode having a sufficient reaction areais obtained. If the penetration resistance is equal to or smaller thanthe upper limit value of the above range, the conductivity of the carbonelectrode is improved and the battery characteristics when the carbonelectrode is used for a battery is further improved.

The penetration resistance of the carbon electrode is a resistance valueof the electrode per unit area in the thickness direction and ismeasured using the following method. A circular sample piece of 10.2 cm²punched from a carbon electrode is disposed between a pair of conductiveplates, the voltage when a current of 10 mA flows while the conductiveplates are pressed with 1020 N from above and below is measured, and theresistance value per unit area of the carbon electrode is calculated. Itis possible to use an ohmmeter to measure the resistance value.

The amount of Ag to be supported in the carbon electrode may be adjustedso that the penetration resistance is within the above range and ispreferably 0.1 to 10 parts by mass, and more preferably 0.1 to 5.0 partsby mass with respect to 100 parts by mass of the total mass of thecarbon fiber and the resin carbide. If the amount of Ag to be supportedis equal to or larger than the lower limit value of the above range, theconductivity of the carbon electrode is improved and the batterycharacteristics when the carbon electrode is used for a battery isfurther improved. If the amount of Ag to be supported is equal to orsmaller than the upper limit value of the above range, it is difficultto close a path along which the electrolytic solution diffuses.

In the present invention, either or both of a Ti oxide and a Sn oxidemay be present on the surface of the porous carbon electrode. That is tosay, either or both of a Ti oxide and a Sn oxide may be present on thesurfaces of the carbon fiber and the resin carbide forming the porouscarbon electrode. The presence of the Ti oxide and the Sn oxide improvesthe hydrophilicity of the electrode and further increases the reactionactivity of the electrode. Thus, the battery characteristics when thecarbon electrode is used for a battery is further improved.

In the porous carbon electrode, either or both of the Ti oxide and theSn oxide may be present uniformly in the thickness direction or may beunevenly distributed in the surface layer.

For example, as will be described later, when the porous carbonelectrode is impregnated in a dispersion liquid having the Ti oxide andthe Sn oxide dispersed therein and is subjected to a heat treatment, theTi oxide and the Sn oxide can be present on the surfaces of the carbonfiber and the resin carbide of the porous carbon electrode.

Examples of the Ti oxide include TiO₂.

Examples of the Sn oxide include SnO and SnO₂ and the Sn oxide ispreferably SnO₂.

The amounts of the Ti oxide and the Sn oxide to be supported in thecarbon electrode are more preferably 0.1 to 10 parts by mass, and stillmore preferably 0.1 to 5 parts by mass with respect to 100 parts by massof the total mass of the carbon fiber and the resin carbide. If theamounts of the Ti oxide and the Sn oxide to be supported are equal to orlarger than the lower limit value of the above range, the hydrophilicityof the carbon electrode is improved and the battery characteristics whenthe carbon electrode is used for a battery is further improved. If theamounts of the Ti oxide and the Sn oxide to be supported are equal to orsmaller than the upper limit value of the above range, it is difficultto close a path along which the electrolytic solution diffuses.

(Manufacturing Method)

Examples of a manufacturing method of a carbon electrode of the presentinvention include a method including the following Steps (1) to (5):

Step (1): a step of forming carbon fibers and a binder fiber into asheet to obtain a carbon fiber sheet;

Step (2): a step of impregnating the carbon fiber sheet in athermosetting resin to obtain a resin-impregnated carbon fiber sheet;

Step (3): a step of subjecting the resin-impregnated carbon fiber sheetto heat press processing to obtain a resin-cured carbon fiber sheet;

Step (4): a step of subjecting the resin-cured carbon fiber sheet tocarbonization at 400 to 3000° C. in an inert atmosphere to obtain aporous carbon electrode; and

Step (5): a step of carrying out a hydrophilization treatment bytransporting the porous carbon electrode and subjecting the porouscarbon electrode to a heat treatment while causing an oxidizing gas at400 to 800° C. to flow in a direction perpendicular to a direction inwhich the porous carbon electrode is transported.

In Step (1), the carbon fiber sheet is formed using the carbon fibersand the binder fiber. It is possible to easily form the carbon fibersinto a sheet when the binder fiber is used together.

Examples of the binder fiber include polyvinyl alcohol (PVA) fiber,polyvinyl acetate fiber, polyethylene fiber, polyethylene terephthalate(PET) fiber, and polyethylene (PE) pulp. Among them, PVA fiber andpolyethylene fiber are preferable because they have an excellent bindingforce and can more effectively prevent carbon fibers from falling off.As the binder fiber, one kind may be used independently or a combinationof two or more kinds may be used.

The amount of the binder fiber to be used is preferably 5 to 100 partsby mass, preferably 10 to 100 parts by mass, more preferably 40 to 100parts by mass, and particularly preferably 15 to 60 parts by mass withrespect to 100 parts by mass of the total mass of the carbon fibers. Ifthe amount of the binder fiber to be used is equal to or larger than thelower limit value of the above range, it is possible to more effectivelyprevent the carbon fibers from falling off. If the amount of the binderfiber to be used is equal to or smaller than the upper limit value ofthe above range, a carbon fiber sheet having sufficient strength can beobtained. Furthermore, a carbon electrode having high liquidpermeability can be obtained.

Examples of a method of forming carbon fibers and a binder fiber into asheet include a wet method of dispersing carbon fibers and a binderfiber in a liquid medium and making paper and a paper making method suchas a dry method in which carbon fibers and a binder fiber are dispersedin air and piled up. A wet method is preferable in view of sheetstrength and fiber dispersion uniformity.

A liquid medium may be any medium in which carbon fibers and a binderfiber are not dissolved, examples thereof include water and organicsolvents such as methanol, ethanol, ethylene glycol, and propyleneglycol, and water is preferable in view of productivity.

The carbon fiber sheet may be manufactured using a continuous method ora batch method. The continuous method is preferable in view of theproductivity and the mechanical strength of the carbon fiber sheet.

The basis weight of the carbon fiber sheet is preferably 10 to 200 g/m².

The thickness of the carbon fiber sheet is preferably 100 to 2000 μm.

It is desirable to subject the carbon fiber sheet to an entanglementtreatment to form a carbon fiber sheet having an entangled structure.

Examples of the entanglement treatment include a machine entanglementmethod such as a needle punching method, a high-pressure liquid jetmethod such as a water jet punching method, and a high-pressure gasinjection method such as a steam jet punching method. The high-pressureliquid jet method is preferable because it is possible to easilyminimize the breakage of carbon fibers due to the entanglement treatmentand easily obtain an appropriate entanglement property.

In the high-pressure liquid jet method, for example, a carbon fibersheet is placed above a support member having a substantially smoothsurface, a liquid columnar flow, a liquid fan flow, a liquid slit flow,and the like to be injected at a pressure of 1 MPa or more act on thecarbon fiber sheet, and fibers in the carbon fiber sheet are entangled.As the support member having a substantially smooth surface, it ispossible to use a support member in which a pattern of the supportmember is not formed in the carbon fiber sheet which has been subjectedto the entanglement treatment and from which injected liquid is quicklyremoved. Specific examples thereof include a 30 to 200 mesh wire net, aplastic net, or a roll.

After the carbon fiber sheet is manufactured above the support memberhaving a substantially smooth surface, subsequently, continuouslyperforming the entanglement treatment using the high-pressure liquid jetmethod or the like is preferable in view of the productivity.

A liquid used in the high-pressure liquid jet method may be any liquidwhich does not dissolve fibers to be treated and water and deionizedwater are preferable. Water may be hot water.

In the case of a columnar flow, a nozzle hole diameter of ahigh-pressure liquid jet nozzle is preferably 0.06 to 1.0 mm, and morepreferably 0.1 to 0.3 mm.

The distance between a nozzle injection hole and a carbon fiber sheetduring processing is preferably 0.5 to 5 cm.

The pressure of a liquid is preferably 1 MPa or more, and morepreferably 1.5 mPa or more in view of the entanglement of fibers.

The entanglement treatment is repeatedly performed multiple times. Forexample, after a carbon fiber sheet is subjected to the entanglementtreatment, a carbon fiber sheet may be further laminated on the treatedcarbon fiber sheet and subjected to the entanglement treatment. Inaddition, after a carbon fiber sheet is subjected to the entanglementtreatment from one surface side, the carbon fiber sheet may be turnedupside down and entangled again from the opposite side.

After the carbon fiber sheet is subjected to the entanglement treatmentusing the high-pressure liquid jet method, the carbon fiber sheet may bedried. The drying method is not particularly limited and examplesthereof include a heat treatment using a high-temperature atmospherefurnace or a far-infrared heating furnace and a heat treatment using ahot plate, a hot roll, or the like.

The drying temperature can be, for example, 20 to 200° C.

The drying time can be, for example, 1 minute to 24 hours.

In Step (2), for example, a resin-impregnated carbon fiber sheet isobtained by impregnating a carbon fiber sheet in a resin compositioncontaining a thermosetting resin or a thermosetting resin and othercomponents such as a carbon powder.

An impregnation method is not particularly limited and examples thereofinclude a method of applying a dispersion liquid having a thermosettingresin or a resin composition dispersed therein on a surface of a carbonfiber sheet and a method supplying a dispersion liquid to a carbon fibersheet through a dip-nip method using a squeezing device. A method inwhich a resin film made of a thermosetting resin or a resin compositionand a carbon fiber sheet overlap, are heated, are pressed, and aresubjected to transferring may be used.

As a dispersion medium, water, alcohol, dimethylformamide, anddemethylacetamide are preferable in view of handleability and productioncosts. When water is used as the dispersion medium, a dispersant such asa surfactant may be used to disperse a resin and a carbon powder.

The amount of a thermosetting resin to be impregnated in a carbon fibersheet is preferably 50 to 200 parts by mass, and more preferably 80 to150 parts by mass with respect to 100 parts by mass of a carbon fibersheet.

An impregnation treatment may be repeatedly performed multiple times.For example, after a carbon fiber sheet is subjected to the impregnationtreatment, a carbon fiber sheet may be laminated on the treated carbonfiber sheet and subjected to the impregnation treatment. In addition,after the carbon fiber sheet is subjected to the impregnation treatmentfrom one surface side, the carbon fiber sheet may be turned upside downand subjected to the impregnation treatment again from the oppositeside.

After the resin-impregnated carbon fiber sheet is subjected to theimpregnation treatment, the resin-impregnated carbon fiber sheet may bedried. The drying method is not particularly limited and examplesthereof include a heat treatment using a high-temperature atmospherefurnace or a far-infrared heating furnace and a heat treatment using ahot plate, a hot roll, or the like.

The drying temperature can be, for example, 40 to 120° C.

The drying time can be, for example, 0.1 minutes to 24 hours.

In Step (3), a resin-cured carbon fiber sheet is obtained by subjectinga resin-impregnated carbon fiber sheet to heat press processing andcuring a thermosetting resin in the resin-impregnated carbon fibersheet.

Examples of a method of subjecting a resin-impregnated carbon fibersheet to heat press processing include a method of hot-pressing aresin-impregnated carbon fiber sheet from both upper and lower sidesusing smooth rigid plates and a method of performing hot-pressing usinga continuous belt press device. It is desirable that the continuous beltpress device be used as heat press processing of a resin-impregnatedcarbon fiber sheet in the case of the continuous method.

Examples of a pressing method in the continuous belt press deviceinclude a method of applying linear pressure to a belt using a rollpress and a method of performing pressing through surface pressure usinga hydraulic head press. A method of performing pressing through surfacepressure using a hydraulic head press because a smoother carbonelectrode can be obtained.

At the time of hot pressing, it is desirable to apply a release agentapplied to a rigid plate or a belt in advance so that a thermosettingresin or the like is not attached or arrange release paper between theresin-impregnated carbon fiber sheet and a rigid plate and a belt.

Although the heating temperature at the time of hot pressing alsodepends on a curing temperature of a thermosetting resin, the heatingtemperature is preferably 100 to 400° C., more preferably 150 to 380°C., and still more preferably 180 to 360° C. If the heating temperatureis equal to or higher than the lower limit value of the above range, thethermosetting resin is easily cured. If the heating temperature is equalto or lower than the upper limit value of the above range, it is easy toprevent the thermosetting resin and the binder fiber from burning out.

The pressure to be pressed at the time of hot pressing is preferably 1to 20 MPa, and more preferably 5 to 15 MPa. If the pressure is equal toor higher than the lower limit value of the above range, it is possibleto easily obtain a carbon electrode having a smooth surface. If thepressure is equal to or lower than the upper limit value of the aboverange, a carbon fiber is not easily broken.

The hot pressing time is preferably 0.1 to 5.0 minutes, and morepreferably 0.1 to 2.0 minutes.

In Step (4), a porous carbon electrode is obtained by subjecting aresin-cured carbon fiber sheet to carbonization.

The carbonization of the resin-cured carbon fiber sheet is performed inan inert atmosphere to enhance the conductivity of the carbon electrode.It is desirable to use an inert gas such as nitrogen or argon.

The heating temperature at the time of a carbonization treatment is 400to 3000° C., preferably 600 to 2500° C., and more preferably 1000 to2300° C. If the heating temperature at the time of a carbonizationtreatment is equal to or higher than the lower limit value of the aboverange, it is possible to easily obtain a carbon electrode havingexcellent conductivity. If the heating temperature at the time of acarbonization treatment is equal to or lower than the upper limit valueof the above range, it is possible to obtain a carbon electrode havingexcellent reactivity.

The carbonization treatment time can be 1 minutes to 1 hour.

In Step (4), it is desirable to impregnate a porous carbon electrode ina dispersion liquid of a metal oxide to further support the metal oxide.Thus, the above-described non-through holes are formed in the carbonelectrode and the specific surface area is increased.

Examples of the metal oxide include Ag₂O, a Ti oxide (TiO₂), and a Snoxide (SnO and SnO₂). As the metal oxide, one kind may be usedindependently or a combination of two or more kinds may be used.

Examples of the dispersion medium configured to disperse the metal oxideinclude water and alcohol. Water may be deionized water. When water isused as the dispersion medium, a dispersant such as a surfactant may beused.

The amount of the metal oxide in the dispersion liquid is preferably 0.5to 20% by mass, and more preferably 0.5 to 10% by mass with respect tothe total mass of the dispersion liquid.

The amount of a metal oxide to be supported in a porous carbon electrodehaving the metal oxide supported therein is preferably 0.1 to 10 partsby mass, and more preferably 0.1 to 5 parts by mass with respect to 100parts by mass of the total mass of carbon fibers and a resin carbide.

The impregnation treatment of the dispersion liquid of the metal oxidemay be repeatedly performed multiple times. For example, after a porouscarbon electrode is subjected to the impregnation treatment, a porouscarbon electrode may be further laminated on the treated porous carbonelectrode and subjected to the impregnation treatment. In addition,after the porous carbon electrode is subjected to the impregnationtreatment from one surface side, the porous carbon electrode may beturned upside down and subjected to the impregnation treatment againfrom the opposite side.

After the porous carbon electrode is subjected to the impregnationtreatment, the porous carbon electrode having the metal oxide supportedtherein may be dried. The drying method is not particularly limited andexamples thereof include a heat treatment using a high-temperatureatmosphere furnace or a far-infrared heating furnace and a heattreatment using a hot plate, a hot roll, or the like.

The drying temperature of the porous carbon electrode having the metaloxide supported therein can be, for example, 90 to 300° C.

The drying time of the porous carbon electrode having the metal oxidesupported therein can be, for example, 0.5 minutes to 2 hours.

In Step (5), a porous carbon electrode is transported and is subjectedto a heat treatment while causing an oxidizing gas at 400 to 800° C. toflow in a direction perpendicular to a direction in which the porouscarbon electrode is transported. Thus, the porous carbon electrode issubjected to hydrophilization and a carbon electrode in which theconditions such as the contact angle of water are satisfied is obtained.For example, a method in which the porous carbon electrode istransported, passes through a furnace in an air atmosphere of 400 to800° C., and is subjected to a heat treatment while causing an oxidizinggas to flow in a direction perpendicular to a direction in which theporous carbon electrode is transported in the furnace is an exemplaryexample.

Examples of the oxidizing gas include oxygen, air, and carbon dioxide.Among them, air is preferable in view of use costs. As the oxidizinggas, one kind may be used independently or a combination of two or morekinds may be used.

A temperature in a furnace is 400 to 800° C., preferably 500 to 750° C.,and more preferably 550 to 700° C. If the temperature in a furnace isequal to or higher than the lower limit value of the above range, it ispossible to easily obtain a carbon electrode having excellenthydrophilicity and high reaction activity in a battery. If thetemperature in a furnace is equal to or lower than the upper limit valueof the above range, a carbon electrode having sufficient strength can beobtained.

A heat treatment time is preferably 5 to 60 minutes, more preferably 0.5to 10 minutes, and still more preferably 0.5 to 5 minutes. If the heattreatment time is equal to or larger than the lower limit value of theabove range, it is possible to easily obtain a carbon electrode havingexcellent hydrophilicity. If the heat treatment time is equal to orsmaller than the upper limit value of the above range, high productivityis provided.

When Ag₂O is used as the metal oxide in Step (4), a reduction reactionrepresented by 2Ag₂O→4Ag+O₂ occurs during a heat treatment, Ag isdeposited on a surface of the carbon fiber and the resin carbide, and acarbon electrode having excellent conductivity can be obtained.

Also, Ag₂O acts as an oxidizing agent and a functional group such as acarboxy group and a hydroxy group is introduced into carbon fibers. Inaddition, non-through holes are formed in surfaces of the carbon fibersand the specific surface area of the porous carbon electrode is furtherincreased. Thus, since the functional group is introduced at a highdensity in the carbon electrode, the reaction activity of the electrodeis further increased. The formation of the non-through holes in thesurface of the carbon fiber is presumed to be caused by the reductionreaction.

When a Ti oxide (TiO₂) or a Sn oxide (SnO and SnO₂) is used as the metaloxide in Step (4), the metal oxide is present on surfaces of carbonfibers and a resin carbide of the carbon electrode to be obtained and acarbon electrode having more excellent hydrophilicity can be obtained.

As the hydrophilization treatment, a method in which, in addition to aheat treatment using an oxidizing gas, a porous carbon electrode isimmersed in a nitric acid solution, subjected to an electrolytictreatment, washed with water, and dried may be adopted. In this case,the electrolytic treatment may be performed after the heat treatmentusing an oxidizing gas is performed or the heat treatment using anoxidizing gas may be performed after the electrolytic treatment isperformed.

Examples of the nitric acid solution include an aqueous solution havinga nitric acid concentration of 0.1 to 13.0 mol/L and the nitric acidconcentration is preferably 0.5 to 5 mol/L.

The temperature of the nitric acid solution at the time of anelectrolytic treatment is preferably 5 to 40° C., and more preferably 10to 30° C. If the temperature is equal to or higher than the lower limitvalue of the above range, it is possible to easily obtain a carbonelectrode having excellent hydrophilicity. If the temperature is equalto or lower than the upper limit value of the above range, highproductivity is provided.

The treatment time of the electrolytic treatment is preferably 0.1 to5.0 minutes, and more preferably 0.5 to 3.0 minutes. If the treatmenttime is equal to or larger than the lower limit value of the aboverange, it is possible to easily obtain a carbon electrode havingexcellent hydrophilicity. If the treatment time is equal to or smallerthan the upper limit value of the above range, high productivity isprovided.

As the hydrophilization treatment, in addition to the heat treatmentusing an oxidizing gas or the heat treatment using an oxidizing gas andthe electrolytic treatment, an atmospheric pressure plasma treatment maybe performed on both surfaces of the porous carbon electrode. The orderof the atmospheric pressure plasma treatment is not particularlylimited, may be performed before the heat treatment or the electrolytictreatment, may be performed after the heat treatment or the electrolytictreatment, or may be performed between the heat treatment and theelectrolytic treatment.

The atmospheric pressure plasma treatment may be a direct method or aremote method. The direct method is a method in which a porous carbonelectrode is arranged between two flat plate electrodes arranged inparallel with each other and is subjected to processing. The remotemethod is a method in which plasma generated between electrodes issprayed onto a porous carbon electrode and is subjected to processing.

In the plasma treatment, an introduction gas to be introduced into aplasma treatment chamber may be any gas as long as a plasma gas isstably generated. In addition, a gas containing an inert gas in a rangeof 97.0% by volume or more and 99.99% by volume or less and an activegas in a range of 0.01% by volume or more and 3.0% by volume or less ispreferable.

Examples of the Inert Gas Include Nitrogen and Argon.

Examples of the Active Gas Include Oxygen, Hydrogen, and CarbonMonoxide.

As described above, in the carbon electrode of the present invention,all of the contact angles θ_(A) of water on both surfaces of the carbonelectrode in the thickness direction and the contact angle θ_(B) ofwater in the middle portion of the carbon electrode in the thicknessdirection are controlled within specific ranges. In this way, when thecontact angle of water is low and excellent hydrophilicity is providednot only on the surface of the carbon electrode but also on the middleportion of the carbon electrode in the thickness direction, highreaction activity of the electrode is provided over the entire body inthe thickness direction. For this reason, when the carbon electrode ofthe present invention is used for a redox flow battery, excellentbattery characteristics can be obtained.

Also, in the carbon electrode of the present invention, the reactionactivity of the electrode is further increased by controlling thethickness, the basis weight, and the specific surface area withinspecific ranges.

Although the present invention will be described in detail below withreference to Examples, the present invention is not limited by thefollowing description.

[Measurement of Contact Angle of Water]

In contact angles θ_(A) of water on both surfaces of a carbon electrodein a thickness direction, a contact angle of water per sample wasmeasured at a total of 10 points, i.e., 5 points on each of a frontsurface and a rear surface using a contact angle meter (manufactured byMatsubo Co., Ltd.) and an average value thereof was used as the contactangles θ_(A). Measurement positions were set at 5 points which were evenin a sheet width direction. In a contact angle θ_(B) of a middle portionof the carbon electrode in the thickness direction, a double-sided tapewas applied on both surfaces of the carbon electrode, the double-sidedtape was peeled off to expose a surface of the carbon electrode in themiddle portion thereof in the thickness direction, the contact angle ofwater was measured at 10 points per sample, and the average valuethereof was used as the contact angle θ_(B). A position of the surfaceof the carbon electrode in the middle portion thereof in the thicknessdirection to be used for the measurement was a portion at a position of0.4 to 0.6 with respect to a thickness 1 of the carbon electrode.

[Evaluation of Battery Characteristics]

Battery characteristics of a redox flow battery using a carbon electrodeof each example were measured as follows.

The carbon electrode was cut into an electrode area of 9 cm² with 3 cmin an upward/downward direction (a liquid passage direction) and 3 cm ina width direction and a cell was assembled. When a carbon electrodehaving a thin thickness is used so that the thickness of the carbonelectrode was 0.60 mm, adjustment was performed in accordance with thenumber of sheets to be used. Nafion 212 membrane was used as an ionexchange membrane. Charging was performed up to 1.45 V at 70 mA/cm² anddischarge characteristics were confirmed using an electrochemicalmeasuring device manufactured by Solartron. The results are shown inTables 1 and 2 as maximum power densities. Furthermore, a 3.0 mol/Laqueous sulfuric acid solution of 1.0 mol/L vanadium oxysulfate was usedas a positive electrode electrolytic solution and 3.0 mol/L aqueoussulfuric acid solution of 1.0 mol/L vanadium sulfate was used as anegative electrode electrolytic solution. The amount of electrolyticsolution was set to a large excess with respect to a cell and piping.The liquid flow rate was 20 mL/min and was measured at 30° C.

[Specific Surface Area]

The specific surface area of a carbon electrode was measured through amercury injection method using a mercury porosimeter.

[Measurement of Penetration Resistance]

The penetration resistance of a carbon electrode was measured throughthe following method.

A circular sample piece of 10.2 cm² punched outside of the carbonelectrode was arranged between a pair of conductive plates and a voltagewhen a current of 10 mA was applied while applying a pressure of 1020 Nfrom above and below the conductive plates was measured and calculatedas a resistance value in a unit area of the carbon electrode.

A low resistance meter (Agilent 34420A manufactured by Agilent) was usedas a measuring device.

Example A1

A dispersion liquid was prepared by uniformly dispersing 100 parts bymass of carbon fibers (PAN-based carbon fibers; an average fiberdiameter of 7 μm; and an average fiber length of 3 mm) in water,disintegrating the carbon fibers into single fibers, when sufficientdispersion was determined to be performed, and uniformly dispersing 80parts by mass of polyethylene pulp (manufactured by Mitsui Chemicals,Inc.; trade name SWP) and 20 parts by mass of PVA fiber (manufactured byKuraray Co., Ltd.; “VPB105-1;” an average fiber length: 3 mm) as binderfibers.

Paper was made from the dispersion liquid and then dried to obtain acarbon fiber sheet with a basis weight of 40 g/m².

A resin-impregnated carbon fiber sheet with a basis weight of 60 g/m²was obtained by impregnating a carbon fiber sheet in a methanol solutioncontaining 30% by mass of a phenolic resin (manufactured by DICCorporation; “Phenolite J-325”) as a thermosetting resin and drying thecarbon fiber sheet at 40° C. for 15 minutes using a hot air dryer.

A resin-cured carbon fiber sheet was obtained by subjecting theresin-impregnated carbon fiber sheet to heat press processing under theconditions such as 250° C. and 10 MPa for 1 minutes using a double beltpress machine.

A porous carbon electrode with a thickness of 200 μm was obtained bysubjecting the resin-cured carbon fiber sheet to carbonization in acarbonization furnace through heating at 2000° C. in a nitrogen gasatmosphere for 15 minutes.

A hydrophilic porous carbon electrode was obtained by passing the porouscarbon electrode in a furnace at 680° C. in an air atmosphere whiletransporting the porous carbon electrode and subjecting the porouscarbon electrode to a heat treatment for 3 minutes while causing air toflow in a direction perpendicular to a direction in which the porouscarbon electrode is transported in the furnace.

Contact angles θ_(A) of water on both surfaces of the obtainedhydrophilic porous carbon electrode in the thickness direction were 3°and 4° and a contact angle θ_(B) of water in a middle portion of theobtained hydrophilic porous carbon electrode in the thickness directionwas 5°.

Example A2

A porous carbon electrode was obtained in the same manner as in ExampleA1.

A hydrophilic porous carbon electrode was obtained by passing the porouscarbon electrode at 680° C. in an air atmosphere in a furnace whiletransporting the porous carbon electrode, subjecting the porous carbonelectrode to a heat treatment for 3 minutes while causing air to flow ina direction perpendicular to a direction in which the porous carbonelectrode is transported in the furnace, and further subjecting theporous carbon electrode to an atmospheric pressure plasma treatment. Inthe atmospheric pressure plasma treatment, a mixed gas ofnitrogen:oxygen=99.99:0.0100 (% by volume) was used as an introductiongas into a plasma treatment chamber of an atmospheric pressure plasmadevice AP-T03-S230 (Sekisui Chemical Co., Ltd.) and introduced at 75L/min. The porous carbon electrode was subjected to a plasma treatmentat an output of 375 W for 0.5 seconds in a state in which an ejectionport of a plasma gas of the plasma device was placed 3 mm away from asurface of the porous carbon electrode so that the plasma gas was blownto the porous carbon electrode from a perpendicular direction of theporous carbon electrode. This treatment was performed on both surfacesof the porous carbon electrode.

Contact angles θ_(A) of water on both surfaces of the obtainedhydrophilic porous carbon electrode in the thickness direction are 2°and 3° and a contact angle θ_(B) of water on a middle portion of theobtained hydrophilic porous carbon electrode in the thickness directionis 5°.

Comparative Example A1

A porous carbon electrode was obtained in the same manner as in ExampleA1.

The porous carbon electrode was immersed in an aqueous solution (25° C.)with a nitric acid concentration of 3 mol/L for 2 minutes, subjected toan electrolytic treatment, washed with water, and then dried.

Contact angles θ_(A) of water on both surfaces of the obtained porouscarbon electrode in the thickness direction are 32° and 34° and acontact angle θ_(R) of water in a middle portion of the porous carbonelectrode in the thickness direction is 38°.

Comparative Example A2

A porous carbon electrode was obtained in the same manner as in ExampleA1.

The porous carbon electrode was immersed in an aqueous solution (25° C.)with a nitric acid concentration of 3 mol/L for 2 minutes, subjected toan electrolytic treatment, washed with water, dried, and then subjectedto an atmospheric pressure plasma treatment. In the atmospheric pressureplasma treatment, a mixed gas of nitrogen:oxygen=99.99:0.0100 (% byvolume) was used as an introduction gas into a plasma treatment chamberof an atmospheric pressure plasma device AP-T03-S230 (Sekisui ChemicalCo., Ltd.) and introduced at 75 L/min. The porous carbon electrode wassubjected to a plasma treatment at an output 375 W for 0.5 seconds in astate in which an ejection port of a plasma gas of a plasma device wasplaced 3 mm away from both surfaces of the porous carbon electrode sothat the plasma gas was blown to the porous carbon electrode from aperpendicular direction of the porous carbon electrode. Contact anglesθ_(A) of water on both surfaces of the obtained porous carbon electrodein the thickness direction were 19° and 18° and a contact angle θ_(B) ofwater in a middle portion of the obtained porous carbon electrode in thethickness direction was 25°.

Comparative Example A3

A porous carbon electrode was obtained in the same manner as in ExampleA1 except that the porous carbon electrode was not subjected to ahydrophilization treatment.

Table 1 shows the evaluation results of the battery characteristics ofthe redox flow battery using the carbon electrode of each example.

TABLE 1 Contact angle of water [°] Output density θ_(A) θ_(B) [mW/cm²]Example A1 3 4 5 1125 Example A2 2 3 5 1165 Comparative Example A1 32 3438 890 Comparative Example A2 19 18 25 926 Comparative Example A3 98 104100 420

As shown in Table 1, redox flow batteries using the hydrophilic porouscarbon electrodes of Examples A1 and A2 in which all of the contactangles θ_(A) and the contact angle θ_(B) were within a range of 0 to 15°had excellent battery characteristics.

Redox flow batteries using the porous carbon electrodes of ComparativeExamples A1 to A3 in which at least one of the contact angles θ_(A) andthe contact angle θ_(B) was outside of a range of 0 to 15° had inferiorbattery characteristics.

Example B1

A dispersion liquid was prepared by uniformly dispersing 100 parts bymass of carbon fibers (PAN-based carbon fibers; an average fiberdiameter of 7 μm and an average fiber length of 3 mm) in water,disintegrating the carbon fibers into single fibers, when sufficientdispersion was determined to be performed, and uniformly dispersing 80parts by mass of polyethylene pulp (manufactured by Mitsui Chemicals,Inc., trade name SWP) and 20 parts by mass of PVA fiber (manufactured byKuraray Co., Ltd.; “VPB105-1;” an average fiber length of 3 mm) asbinder fibers.

Paper was made from the dispersion liquid and then dried to obtain acarbon fiber sheet with a basis weight of 40 g/m².

A resin-impregnated carbon fiber sheet with a basis weight of 60 G/m²was obtained by immersing a carbon fiber sheet in a dispersion liquidobtained by dispersing 30% by mass of a phenolic resin (manufactured byDIC Corporation; “Phenolite J-325”) in methanol as a thermosetting resinto impregnate the carbon fiber sheet in the thermosetting resin anddrying the carbon fiber sheet at 60° C. for 15 minutes using a hot airdryer.

A resin-cured carbon fiber sheet was obtained by subjecting theresin-impregnated carbon fiber sheet to heat press processing under theconditions such as 250° C. and 10 mPa for 1 minutes using a double beltpress machine.

A porous carbon electrode with a thickness of 210 μm was obtained bysubjecting the resin-cured carbon fiber sheet to carbonization in acarbonization furnace through heating at 2000° C. in a nitrogen gasatmosphere for 15 minutes.

The porous carbon electrode was immersed in a dispersion liquid obtainedby dispersing 10% by mass of Ag₂O (manufactured by NACALAI TESQUE, INC.)in water to impregnate Ag₂O and dried at 100° C. for 1 hour using a hotair dryer.

A hydrophilic porous carbon electrode was obtained by passing the porouscarbon electrode having Ag₂O supported therein in a furnace at 400° C.in an air atmosphere while transporting the porous carbon electrode andsubjecting the porous carbon electrode to a heat treatment for 10minutes while causing air to flow in a direction perpendicular to adirection in which the porous carbon electrode is transported in thefurnace. The thickness of the obtained hydrophilic porous carbonelectrode was 0.21 mm and a basis weight thereof was 57 g/m², and thespecific surface area thereof was 6.6 m²/g.

Example B2

A hydrophilic porous carbon electrode was prepared in the same manner asin Example B1 except that amounts of carbon fibers, binder fibers, and athermosetting resin to be used were adjusted and a porous carbon sheetwith a thickness of 630 μm was used. The thickness of the obtainedhydrophilic porous carbon electrode was 0.63 mm, the basis weightthereof was 178 g/m², and the specific surface area thereof was 7.2m²/g.

Comparative Example B1

A porous carbon electrode was prepared by performing the dispersing tothe heat treatment in the same manner as in Example B1 except that TiO₂was used instead of Ag₂O. The thickness of the obtained porous carbonelectrode was 0.21 mm, the basis weight thereof was 65 g/m², and thespecific surface area thereof was 1.2 m²/g.

Comparative Example B2

A porous carbon electrode was prepared by performing the dispersing tothe heat treatment in the same manner as in Example B1 except that SnO₂was used instead of Ag₂O. The thickness of the obtained porous carbonelectrode was 0.21 mm, the basis weight thereof was 66 g/m², and thespecific surface area thereof was 2.4 m²/g.

Comparative Example B3

A porous carbon electrode was obtained in the same manner as in ExampleB1 except that an impregnation treatment and a heat treatment of adispersion liquid of Ag₂O were not performed. The thickness of theobtained porous carbon electrode was 0.21 mm, the basis weight thereofwas 62 g/m², and the specific surface area thereof was 0.9 m²/g.

Table 2 shows the measurement results of thickness, basis weight,specific surface area, contact angles θ_(A), a contact angle θ_(B), anda penetration resistance of a porous carbon electrode of each exampleand the evaluation results of the battery characteristics of a redoxflow battery using the porous carbon electrode.

TABLE 2 Porous carbon electrode Specific Type of Basis surfacePenetration Contact angle of Output metal Thickness weight arearesistance water [°] density oxide [mm] [g/m²] [m²/g] [mΩ · cm²] θ_(A)θ_(B) [mW/cm²] Example B1 Ag₂O 0.21 57 6.6 3.5 5 4 9 1100 Example B2Ag₂O 0.63 178 7.2 5.9 9 8 10 1123 Comparative TiO₂ 0.21 65 1.2 5.7 34 4338 890 Example B1 Comparative SnO₂ 0.21 66 2.4 5.8 33 37 39 880 ExampleB2 Comparative — 0.21 62 0.9 5.6 98 99 95 420 Example B3

As shown in Table 2, the hydrophilic porous carbon electrode of ExamplesB1 and B2 in which all of the contact angles θ_(A) and the contact angleθ_(B) were within a range of 0 to 15° and the thickness, the basisweight, and the specific surface area were in specific ranges hadexcellent battery characteristics of redox flow batteries.

On the other hand, the porous carbon electrodes of Comparative ExamplesB1 and B2 in which the contact angles θ_(A) and the contact angle θ_(B)were large and Comparative Example B3 in which the contact angles θ_(A)and the contact angle θ_(B) were large and the specific surface area wassmall had inferior battery characteristics of redox flow batteries.

What is claimed is:
 1. A porous carbon electrode which is a porouscarbon electrode sheet in which carbon fibers are bonded using a resincarbide, wherein contact angles θ_(A) of water on both surfaces of theporous carbon electrode in a thickness direction are 0 to 15° and acontact angle θ_(B) of water in a middle portion of the porous carbonelectrode in the thickness direction is 0 to 15°, a specific surfacearea of the porous carbon electrode is 1.0 to 1000 m²/g, and wherein Agis present on a surface of the porous carbon electrode.
 2. The porouscarbon electrode according to claim 1, wherein the contact angles θ_(A)of water on both surfaces of the porous carbon electrode in thethickness direction are 0 to 10° and the contact angle β_(B) of water inthe middle portion of the porous carbon electrode in the thicknessdirection is 0 to 15°.
 3. The porous carbon electrode according to claim1, wherein the porous carbon electrode is a hydrophilic porous carbonelectrode, wherein differences between the contact angles θ_(A) of wateron both surfaces of the hydrophilic porous carbon electrode in thethickness direction and the contact angle β_(B) of water in the middleportion of the hydrophilic porous carbon electrode in the thicknessdirection are 0 to 10°.
 4. The porous carbon electrode according toclaim 1, wherein a basis weight is 30 to 300 g/m² and a thickness is0.10 to 0.80 mm.
 5. The porous carbon electrode according to claim 1,wherein non-through holes with a size of an opening portion of 0.1 to5.0 μm and a depth of 0.01 to 1.0 μm are formed in surfaces of thecarbon fibers.
 6. The porous carbon electrode according to claim 1.wherein a penetration resistance of the porous carbon electrode is 3.0to 6.0 mΩ·cm².
 7. The porous carbon electrode according to claim 1,wherein either or both of a Ti oxide and a Sn oxide are present on asurface of the porous carbon electrode.
 8. A manufacturing method of theporous carbon electrode according to claim 1, comprising the followingSteps (1) to (5): Step (1): a step of forming carbon fibers and a binderfiber into a sheet to obtain a carbon fiber sheet; Step (2): a step ofimpregnating the carbon fiber sheet in a thermosetting resin to obtain aresin-impregnated carbon fiber sheet; Step (3): a step of subjecting theresin-impregnated carbon fiber sheet to heat press processing to obtaina resin-cured carbon fiber sheet; Step (4): a step of subjecting theresin-cured carbon fiber sheet to carbonization at 400 to 3000° C. in aninert atmosphere to obtain a hydrophilic porous carbon electrode; andStep (5): a step of carrying out a hydrophilization treatment bytransporting the porous carbon electrode and subjecting the porouscarbon electrode to a heat treatment while causing an oxidizing gas at400 to 800° C. to flow in a direction perpendicular to a direction inwhich the porous carbon electrode is transported.
 9. The manufacturingmethod of the porous carbon electrode according to claim 8, wherein theporous carbon electrode is subjected to the heat treatment for 0.5 to 60minutes using the oxidizing gas.
 10. The manufacturing method of theporous carbon electrode according to claim 8, wherein, in Step (5), theporous carbon electrode is further in mersed in a nitric acid solution,subjected to an electrolytic treatment, washed with water, and dried.11. The manufacturing method of the porous carbon electrode according toclaim 8, wherein, in Step (5), both surfaces of the porous carbonelectrode are further subjected to an atmospheric pressure plasmatreatment.
 12. The manufacturing method of the porous carbon electrodeaccording to claim 8, wherein, in Step (4), the porous carbon electrodeis further impregnated in a dispersion liquid of a metal oxide tosupport the metal oxide.
 13. A manufacturing method of the porous carbonelectrode, which is a porous carbon electrode sheet in which carbonfibers are bonded using a resin carbide, wherein contact angles θ_(A) ofwater on both surfaces of the porous carbon electrode in a thicknessdirection are 0 to 15 ° and a contact angle θ_(B) of water in a middleportion of the porous carbon electrode in the thickness direction is 0to 15°, and a specific surface area of the porous carbon electrode is1.0 to 1000 m²/g, the method comprising: the following Steps (1) to (5):Step (1): a step of forming carbon fibers and a binder fiber into asheet to obtain a carbon fiber sheet; Step (2): a step of impregnatingthe carbon fiber sheet in a thermosetting resin to obtain aresin-impregnated carbon fiber sheet; Step (3): a step of subjecting theresin-impregnated carbon fiber sheet to heat press processing to obtaina resin-cured carbon fiber sheet; Step (4): a step of subjecting theresin-cured carbon fiber sheet to carbonization at 400 to 3000° C. in aninert atmosphere to obtain a hydrophilic porous carbon electrode; andStep (5): a step of carrying out a hydrophilization treatment bytransporting the porous carbon electrode and subjecting the porouscarbon electrode to a heat treatment while causing an oxidizing gas at400 to 800 ° C to low in a direction perpendicular to a direction inwhich the porous carbon electrode is transported.
 14. The manufacturingmethod of the porous carbon electrode according to claim 13, wherein theporous carbon electrode is subjected to the heat treatment for 0.5 to 60minutes using the oxidizing gas.
 15. The manufacturing method of theporous carbon electrode according to claim 13, wherein, in Step (5), theporous carbon electrode is further immersed in a nitric acid solution,subjected to an electrolytic treatment, washed with water, and dried.16. The manufacturing method of the porous carbon electrode according toclaim 13, wherein, in Step (5), both surfaces of the porous carbonelectrode are further subjected to an atmospheric pressure plasmatreatment.
 17. The manufacturing method of the porous carbon electrodeaccording to claim 13, wherein, in Step (4), the porous carbon electrodeis further impregnated in a dispersion liquid of a metal oxide tosupport the metal oxide.